| The process of how lactide synthesized was studied, aiming to find out an high-powered, low-cost catalyst to turn lactic acid into lactide. The chemical structure of lactide was characterized by means of FTIR (Fourier Transform Infred), capillary melting-point testing. The main works and conclusions are included as follows: (1) Some compounds, such as Alumina,magnesia,antimony oxide,stannous sulfate,stannous chloride,stannous octanoate,zinc acetate,zinc lactate,zinc oxide were determined, which accorded to the principle that what can be applied as a good catalyst in the obverse reaction can be done in the reverse reaction. After a series of experiment, stannous and zinc compounds were selected as catalysts in further experiments. (2) Four kind of zinc oxide were prepared respectively by solid state reaction at room temperature, direct precipitation, homogeneous precipitation method, and sol -gel method. All these zinc oxide were utilized as catalyst to prepare lactide in the condition of decompression. Zinc oxide prepared by sol –gel method got a yield rate of 70.1% higher than other methods. Zinc oxide prepared by sol –gel method dispersed more homogeneously in the reactant system. What is more, the price of zinc oxide prepared by sol –gel method is 30000yuan/t, only three times of analysis pure zinc oxide and centesimal of the compound of lanthanum -titanium –zinc. So that zinc oxide prepared by sol –gel method is used in the following experiments. (3) The effects of De-water time&temperature, dehydration temperature&time, temperature of lactide distillation were investigated in detail, resulting in an optimum condition to synthesize lactide with high yield rate. The obtained results show that: Firsl, de-water 1h in the condition of 75~80℃, vacuum of -0.095MPa;then add catalyst with 1% weight of reactant, (stannous octanoate with 1% volume of reactant); the temperature was controlled at 120~125℃to dehydrate 5h; lactide was distiled out at last. Lactide was synthesized in such condition. Lactide was distiled out at 175℃and the highest temperature of distillation was 190℃catalyzed by stannous octanoate, Lactide was distilled out at 180℃and the highest temperature was 200℃, 225℃, 230℃,230℃respectively by stannous chloride,zinc acetate,zinc lactate,zinc oxide. The crude yield rate of lactide was improved. The highest yield rate was 82.0%. But our original intention wasn't obtained. To get this problem solved, the process of lactide synthesis and the theory of how catalyst functioned was discussed in the following section. (4) The models of lactic acid, lactide, chain ester synthesized by two lactic acid, chain ester synthesized by four lactic acid, and lactic acid& stannous system have been calculated with gaussian2003W. According to calculation speed and the characteristics of the model, the method of RHF and the basis 6-31G are utilized. Some structure parameters, such as atomic overlap population, bond distance are calculated. The following results have been concluded:Both the lactic acid oligomer and lactide can be got from Chain ester synthesized by two lactic acid;The distance between carbon atom and oxygen atom at carboxyl in lactic acid and the hydroxyl distance increase when stannous cation is in the system. It is believed that the reaction activity is increased and both of the bond are easily broken. It is presumed that both of the bond in chain ester synthesized by two lactic acid are also more active and these two bond can combined together to produce more lactide when the system has stannous cation; If water produced during the reaction is not removed in time it will prevent the reaction to progress, which is thought as one reason the yield rate of lactide catalyzed by stannous octanoate is only 81.6%. The other reason is that the resultant is not distilled out in time which can also inhibit the reaction to produce lactide. (5) It is believed that zinc oxide functions well on dehydration. Stannous octanoate has some surface activity for its long carbon chain, which assist in distilling out lactide. Zinc oxide and stannous octanoate were used together to synthesize lactide to improve the crude productivity and to validate the former calculation: The experiment was done as the following: zinc oxide was added into the reactant first to dehydrate, then stannous octanoate was added to distill lactide. The crude yield rate is 75% lower than 86.4% was obtained when both zinc oxide and stannous octanoate were added to the reactant system to dehydrate; The rate of 75% was almost the same with 70.1% which obtained when only zinc oxide was used as catalyst. But the highest temperature to distill lactide at the former experiment is lower. It is thought that the added stannous octanoate make it easier to distill lactide, but stannous octanoate was added to the system after dehydration so that the yield rate is not improved; The rate got when both zinc oxide and stannous octanoate added to dehydrate is higher than 81.6% added stannous octanoate alone, It is validated that zinc oxide effected on dehydration; It is showed by detecting the reactant that not only the lactic acid oligomer but also lactide was obtainedafter dehydration; Solid substance was observed at 164℃and plenty of lactide crystal was obtained at 170℃when sulphonate was added to the reactant. The temperature is almost 10℃lower than no sulphonate in the reactant. And the rate of 86.5% was got. It is showed that sulphonate make it easier to distil lactide. The theory was validated and catalyst with high-powered and low-costed was got.
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